Condenser microphones are commonly used in mobile telephones and other consumer electronic devices, embedded systems and other devices. Condenser microphones include microelectromechanical systems (MEMS) microphones, electret condenser microphones (ECMs) and other capacitor-based transducers of acoustic signals. A MEMS microphone element typically includes a conductive micromachined diaphragm that vibrates in response to an acoustic signal. The microphone element also includes a fixed conductive plate parallel to, and spaced apart from, the diaphragm. The diaphragm and the conductive plate collectively form a capacitor. An electrical charge is placed on the capacitor, typically by an associated circuit. The capacitance of the capacitor varies rapidly as the distance between the diaphragm and the plate varies due to the vibration of the diaphragm. Typically, the charge on the capacitor remains essentially constant during these vibrations, so the voltage across the capacitor varies as the capacitance varies. The varying voltage may be used to drive a circuit, such as an amplifier or an analog-to-digital converter, to which the MEMS microphone element is connected. A MEMS microphone element connected to a circuit is referred to herein as a “MEMS microphone system” or a “MEMS system.”
MEMS microphone dies are often electrically connected to application-specific integrated circuits (ASICs) to process the electrical signals from the microphone elements. A MEMS microphone die and its corresponding ASIC are often housed in a common integrated circuit package to keep leads between the microphone element and the ASIC as short as possible, so as to avoid parasitic capacitance caused by long leads, because capacitance coupled to the signal line attenuates the signal from the MEMS microphone element.
When used in consumer electronics devices and other contexts, condenser microphone systems may be subjected to widely varying amplitudes of acoustic signals. For example, a mobile telephone used outdoors under windy conditions or in a subway station subjects the condenser microphone to very loud acoustic signals. Under these circumstances, the diaphragm may reach its absolute displacement limit, and the resulting signal may therefore be “clipped,” causing undesirable distortion. Even if the diaphragm does not reach its absolute displacement limit, the ASIC or other processing circuitry may not be able to handle peaks in the electrical signal from the condenser microphone element due to limited voltage available from a power supply, and the signal may, therefore, be clipped. Clipping can cause a loss of signal contents. For example, if a speech signal is clipped, the output signal waveform becomes flat and no longer varies with the human speech.
U.S. patent application Ser. No. 12/962,136, titled “MEMS Microphone with Programmable Sensitivity,” filed Dec. 7, 2010 (U.S. Pat. Publ. No. 2011/0142261) and U.S. patent application Ser. No. 12/784,143, titled “Switchable Attenuation Circuit for MEMS Microphone System,” filed May 20, 2010 (U.S. Pat. Publ. No. 2010/0310096) disclose circuits for attenuating signals from MEMS microphones.
U.S. Pat. No. 7,634,096, titled “Amplifier Circuit for Capacitive Transducers,” issued Dec. 15, 2009 (U.S. Pat. Publ. No. 2005/0151589) notes a power-on problem in prior art capacitive transducer systems and an associated lack of ability to withstand high-level acoustical signals, such as low-frequency transients generated by door slams or mechanical shocks, etc. The '096 patent discloses a servo-controlled bias circuit for a capacitive transducer, which is said to improve settling of an amplifier circuit coupled to the transducer. The servo-controlled circuit is said to resolve traditionally competing requirements of maintaining a large input resistance of the amplifier circuit to optimize its noise performance and providing fast settling of the amplifier circuit.
Richard S. Burwen, “A Low-Noise High-Output Capacitor Microphone System,” Journal of the Audio Engineering Society, May 1977, Volume 25, Number 5, pages 278-283, describes a capacitor microphone system designed to increase maximum acoustic input capability by including a manual switch to select one of several possible sound pressure levels (SPLs). An amount of feedback within the system is user selectable.
However, the prior art does not disclose or suggest any circuits for automatically selectively attenuating unwanted signals, such as wind buffets.